1. Field of the Invention
The present invention relates to a servo writer for writing servo patterns onto a magnetic tape, more particularly to a servo writer for writing the servo patterns thereonto at a predetermined pitch.
2. Description of the Related Art
One of methods of improving storage density per unit area of a magnetic tape for recording and saving data is to increase the number of data bands for writing data thereon (i.e. to increase the linear density in the width direction of the magnetic tape) In this case, simply relying on a mechanical accuracy of a running mechanism of the magnetic tape has difficulties in securing a complete tracing of a reproducing head over the data bands.
To solve the difficulties, a complete tracing is accomplished by providing the magnetic tape with servo bands thereon in parallel to the data bands so that a recording head and the reproducing head can be servo-controlled in accordance with the servo bands. Note that each servo band is made of a plurality of servo patterns which are written in the running direction at a predetermined pitch therebetween, and which are previously formed on the magnetic tape in a manufacturing process thereof.
In order to perform the above servo control properly, a plurality of servo patterns forming a servo band are needed to be written in such a manner that every pitch therebetween should fall within a predetermined value without variations.
For the sake of a high accurate writing of servo patterns onto a magnetic surface of the magnetic tape at a desired pitch, a special device called as a servo writer has been employed, writing the servo patterns onto the magnetic surface by use of a fixed write head at a constant driving frequency with the magnetic tape kept running at a precisely constant speed. This servo writer is equipped with a tape speed detector for measuring a magnetic tape running speed, and detected values on the running speed measured thereby is fed back to a capstan (tape running drive unit) so that a more precise running speed can be achieved.
In a conventional servo writer, a rotational roller in rotatable contact with a running magnetic tape has been used as a tape speed detector, and the magnetic tape running speed is derived from the rotational speed of the rotational roller, as disclosed in JP-A-2000-048431, for example.
However, there have been the following disadvantages in the conventional servo writer whose rotational roller is for use as a tape speed detector. If slippage occurs between the magnetic surface of the running magnetic tape and the outer circumferential surface of the rotational roller in rotatable contact with this magnetic surface, linear movement of the magnetic tape cannot be converted into a proper rotational movement generated by the rotational roller. This phenomenon can become more apparent if more viscous air is taken up as the magnetic tape runs at a higher speed, resulting in deterioration in accuracy of detected values of the running speed. Variations in diameter of the rotational roller due to abrasion by use or expansion/shrinkage resulting from changes in temperature can also become an error factor of the detected values of the running speed.
If a feed back control for the magnetic tape running speed is based on detected values including such an error, a problem may occur that the pitches between the servo patterns to be written onto the magnetic tape will have values with lower accuracy, departing from the desired value.